1. Field of the Invention
The present invention relates to a multi-color image forming apparatus for use in a digital copier, a laser printer, a laser plotter, a laser facsimile, or in a multifunction product of these units, and more particularly, to a velocity detecting device that detects velocity of an intermediate transfer belt or of a conveyor belt, and a multi-color image forming apparatus, provided with the velocity detecting device, which employs an intermediate transfer system or a direct transfer system.
2. Description of the Related Art
In recent color image forming apparatuses, to meet a request for an increase in their speed, a so-called a tandem system in which photosensitive elements (image carriers) corresponding to toners of four colors (black, cyan, magenta, and yellow) are arranged in parallel is being adopted as a mainstream system. The tandem system requires to finally superimpose toner images of the colors developed on the photosensitive elements on one another on a recording medium such as a paper (e.g., formed paper, post card, cardboard, and OHP sheet). As a system of the superimposition, there are two systems: a direct transfer system of directly superimposing the toner images on the recording medium, and an intermediate transfer belt system of using an intermediate transfer belt to superimpose the toner images of the colors on one another on the intermediate transfer belt and collectively transfer the toner images to the recording medium. If the conveyor belt for feeding the recording medium such as a paper in the direct transfer system and the intermediate transfer belt in the intermediate transfer belt system are not driven with high accuracy, color misregistration will occur.
To drive the intermediate transfer belt with high accuracy, as described in, for example, Japanese Patent Application Laid-open No. 2008-65743, there is known a method of directly forming a mark on the belt, detecting velocity fluctuation of the belt by reading the mark, and feeding back the detected fluctuation to a drive motor, to achieve high-accuracy driving.
However, in the conventional technology described in the Japanese Patent Application Laid-open No. 2008-65743, because the direct formation of the mark on the intermediate transfer belt requires quite a lot of time and effort, productivity is low, and this causes a large cost increase.
Japanese Patent Application Laid-open No. 2003-266828 discloses an implementation example for conveying a paper with high accuracy by detecting a laser speckle from an object to be observed by a two-dimensional image sensor and controlling the drive.
However, the conventional technology described in the Japanese Patent Application Laid-open No, 2003-266828 does not describe an optical system and does not also describe a method of correcting a detection error.
Japanese Patent Application Laid-open No. 2007-283721 discloses an image forming apparatus that calculates a moving velocity of a photosensitive drum from a speckle pattern produced in a reflected light of a laser light irradiated to the photosensitive drum and controls a rotating velocity of the photosensitive drum.
However, the conventional technology described in the Japanese Patent Application Laid-open No. 2007-283721 does not describe a detailed optical system and does not also describe a method of correcting a detection error. Further, the detection is limited to the detection of the photosensitive drum because of the image forming apparatus.
Japanese Patent Kohyo Publication No. 2007-519944 specifies a relationship between each size of a bright patch and a dark patch of a speckle pattern and a size of a pixel of a photodetector, related to a method of generating a cipher for information security, or a cipher which cannot be physically reproduced, by not a number-theoretic algorithm but from a signal indicating detection of light/dark by the photodetector in which a laser light is irradiated to a light-scattering object and a speckle pattern produced thereby is converted to the pixel.
However, the conventional technology described in the Japanese Patent Kohyo Publication No. 2007-519944 provides a detection condition optimized for generating a cipher signal by a speckle pattern, which is different in the purpose from an optimal detection condition for detecting a velocity of a moving object by the speckle pattern described in the present application, and the detection condition cannot easily be analogized.